A monitoring and control system that monitors and controls a target device in a facility such as a power station is conventionally established using a local network. Therefore, security is not regarded as important in the conventional monitoring and control system. However, demands to connect a monitoring and control system to an external network such as the Internet have been increasing in recent years. For example, transmitting process data that indicates a process of control of a target device from a monitoring and control system to a terminal on an external network to use the process data or monitoring a target device from a remote location using a terminal on an external network is demanded.
When a monitoring and control system is connected to an external network, protection of the monitoring and control system against an attack from the external network is needed. A firewall is known as an example of a method of protecting a monitoring and control system. The firewall can theoretically protect the monitoring and control system from an external attack; however, it cannot be asserted that the monitoring and control system is always safe.
Another method of protecting a monitoring and control system other than the firewall is a method that allows communication only in one direction from the monitoring and control system to an external network using a TCP (Transmission Control Protocol)/IP (Internet Protocol). In unidirectional communication using the TCP/IP, process data acquired in the monitoring and control system can be monitored by a terminal on an external network while the monitoring and control system is protected. However, when the process data is to be transmitted from the monitoring and control system to the terminal on the external network, a response from the terminal on the external network is required. Therefore, there is a problem that the process data cannot be transmitted promptly to the terminal on the external network.